Laser Shock Peening of Bulk Metallic Glasses – Part 1 Center for Laser Applications UT Space Institute Tullahoma, TN Midterm Presentation MSE516: Mechanical Metallurgy Deepak Rajput / Web:

 Importance of surface and its relation to failures  Surface improvement techniques  Laser Shock Peening  history, process, layout, examples, etc.  Experiment planned  References Outline 2

Team Members Dr. Hofmeister Director, CLA UT Space Institute Alexander Terekhov Deepak Rajput Dr. Lino Costa Dr. Xiaoxuan Li Center for Laser Applications Kate Lansford 3

Nearly all the fatigue and stress corrosion failures originate at the surface of the component. (long term failures) Surface of a material (or material near the surface) has unique properties. Surface grains deform plastically at lower stress level than those interior grains that are surrounded by neighboring grains because they are subject to less constraint. (a) dislocations terminating at a clean surface will move under a lower stress than those internal dislocations which are anchored at both the ends. (b) minute surface irregularities act as dislocation sources. Due to these reasons mechanical failures mostly occur/initiate on the surface. Hence, surface plays a vital role in the service of a component. Failure: Why & How? 4

In most modes of long-term failure, the denominator is tensile stress. Tensile stresses attempt to stretch or pull the surface apart and may eventually lead to crack initiation. Because crack growth is slowed significantly in a compressive layer, increasing the depth of this layer increases crack resistance. Prevention Idea: Generate compressive stresses on the surface. Failure: Why & How? Cont.. Peening 5

Mechanical working of materials:  Hammer  Shots  Laser Peening: Generation of Compressive Stresses Although obsolete, it is still used today in the hand manufacture of high quality cutting blades 380 mm ball-peen hammer 6

Shot peening is a cold working process in which small spherical media called shot bombard the surface of a part. During the shot peening process, each piece of shot that strikes the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple. To create the dimple, the surface of the material must yield in tension. Below the surface, the material tries to restore its original shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in compression. The overlapping dimples from shot peening create a uniform layer of compressive stress at metal surfaces. Peening: Generation of Compressive Stresses Cont. 7

Shot peening provides considerable increases in part life because cracks do not initiate or propagate in a compressively stressed zone. Compressive stresses are beneficial in increasing resistance to fatigue failures, corrosion fatigue, stress corrosion cracking, hydrogen assisted cracking, fretting, galling and erosion caused by cavitation. The maximum compressive residual stress produced just below the surface of a part by shot peening is at least as great as one-half the yield strength of the material being shot peened. Peening: Generation of Compressive Stresses Cont. 8

Shot peened gear Metal Imrovement Company Paramus, New Jersey Problem: Leaves surface dimpling 9

Peening: Generation of Compressive Stresses Cont. Laser Peening or Laser Shock Peening (LSP): “A process that induces residual compressive stresses on the surface of a component due to shock waves produced by plasma” “Four times deeper than that obtainable from a conventional Shot Peening process” Prototype Laser Peening machines were developed in the 1970s. 10

Laser Shock Peening Historical background: William I. Linlor Hughes Research Laboratories Malibu, California Linlor, W.I. (1962) “Plasma Produced by Laser Bursts” Bull. Am. Phys. Soc. 7, 440 Ready, J.F., (1963) “Development of Plume of Material Vaporized by Giant Pulse Laser”, Appl. Phys. Letters 3 (1), pp # giant pulse investigation, carbon plume speed in air Linlor, W.I. (1963) “Ion Energies Produced by Laser Giant Pulse”, Appl. Phys. Letters 3 (11), pp

Laser Shock Peening 12 Process:  Specimen is coated with a black paint which is opaque to laser beam.  It acts as sacrificial material and is converted to high pressure plasma as it absorbs energy from a high intensity laser (1-10 GW/cm 2 ) for very short time durations (<50 ns).  Generally, specimen is submerged in a transparent media like water or glass plate so that the rapidly expanding plasma cannot escape and the resulting shock wave is transmitted into the specimen subsurface.  Shockwaves thus produced can be much larger than the dynamic yield strength of the material (>1 GPa) and cause plastic deformation to the surface and compressive residual stresses (~1 mm).

Laser Shock Peening 13 Laser used is always a Pulsed Laser  Ability to deliver very high fluence (J/cm 2 ) at shorter time scales (<50 ns)  High repetition rates Spatial resolution Temporal resolution Gaussian Laser Beam Profile

Laser Shock Peening 14 Process layout Taken from Metal Improvement Company website (

Laser Shock Peening 15 Purdue University Dr. Yung C. Shin (Professor) Laser-Assisted Materials Processing Laboratory (LAMPL) School of Mechanical Engineering Purdue University, West Lafayette, Indiana

16 Laser Shock Peening Example: Robot controlled LSP Taken from Metal Improvement Company website (

Laser Shock Peening Laser Shock Peening of 6061-T6 Aluminum Alloy Rubio-Gonzalez C. et al. (2004), “Effect of Laser Shock Processing on Fatigue Crack Growth and Fracture Toughness of 6061-T6 Aluminum Alloy”, Mat. Sci. Eng. – A, 386:

Laser Shock Peening Laser Shock Peening of 6061-T6 Aluminum Alloy Rubio-Gonzalez C. et al. (2004), “Effect of Laser Shock Processing on Fatigue Crack Growth and Fracture Toughness of 6061-T6 Aluminum Alloy”, Mat. Sci. Eng. – A 386: Fatigue crack growth rates with and without LSP under different pulse densities

Laser Shock Peening Laser Shock Peening of 6061-T6 Aluminum Alloy Rubio-Gonzalez C. et al. (2004), “Effect of Laser Shock Processing on Fatigue Crack Growth and Fracture Toughness of 6061-T6 Aluminum Alloy”, Mat. Sci. Eng. – A 386: Load-displacement curves to determine fracture toughness

Laser Shock Peening 20 An increase in fatigue strength is accompanied by:  the creation of large magnitudes of compressive residual stresses and  the increased hardness which develop in the subsurface! The transient shock waves can also induce:  microstructure changes near the surface  cause high density of dislocations to be formed Microstructure changes + Dislocation entanglement = Improved mechanical properties

Problem at hand: BMG 21 Poor tensile ductility of Bulk Metallic Glasses Idea = Control of Residual Stress Genesis: Zhang Y., Wang W.H., and Greer A.L, (2006) “Making Metallic Glasses Plastic by Control of Residual Stress”, Nature Materials, Vol 5, pp  Work- softening  Shear localization BMG studied : Vitreloy 1 Process used: Shot Peening

Planned Experiment 22 Laser Shock Peening of Cu-based Bulk Metallic Glasses Suggested Laser: Excimer laser (λ = 337 nm) Planned Layout:

References 23 Online resource on Metal Improvement Company website Forsyth P.J.E., “The Physical Basis of Metal Fatigue”, American Elsevier Publication Company, Inc., New York: 1969 Linlor W.I (1963). “Ion Energies Produced by Laser Giant Pulse”, Appl. Phys. Lettr., 3(11) Skeen C.H. and York C.H. (1968). “Laser-Induced “Blow-Off” Phenomena”, Appl. Phys. Lettr., 12(11): Anderholm N.C. (1969). “Laser Generated Stress Waves”, Appl. Phys. Lettr., 16(3): Warren A.W., Guo Y.B., and Chen S.C. (2008). “Massive Parallel Laser Shock Peening: Simulation, Analysis, and Validation”, Intl J. Fatigue, 30: Wang H., Xijun S., and Li X. (2003). “Laser Shock Processing of an Austenitic Stainless Steel and a Nickel-base Superalloy”, J. Mater. Sci. Technol., 19(5): Zhang Y., Wang W.H., and Greer A.L, (2006) “Making Metallic Glasses Plastic by Control of Residual Stress”, Nature Materials, Vol 5, pp Rubio-Gonzalez C. et al. (2004), “Effect of Laser Shock Processing on Fatigue Crack Growth and Fracture Toughness of 6061-T6 Aluminum Alloy”, Mat. Sci. Eng. – A, 386:

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